OP, this is not normal or nice behaviour from your relative. You have done amazing and this person is just trying to drag you back into their twisted perception of you of someone they can humiliate.
Some suggestions (other than 'fuck off')
'Its a bit weird you keep going on about this to be honest, let's change the subject! Hows things with you, how's xxxx?'
'Yeah I was actually a really self conscious teenager and your comments about my weight didn't help then, and dont help now, so lets agree to not talk about it! Hows things with you, how's xxx?'
What i find works best with people like this is that they hate being accused of being weird. They HATE it. (I am actually weird - not rude - and I dont mind it, so i find it fascinating how much it unsettles people)
AND/OR boring, they hate being called boring. Or obsessed.
Its a great way to get them to shut up.
'We've discussed this already and its weird you and bringing it up again, if you want advice on weight loss injections im sure there's plenty of forums online you could fine'
'You're being so weird about this! And its a bit boring tbh, let's talk about something else' (changes subject)
'No offence, but this is a bit boring for me, you mentioning my weight all the time, its a bit weird tbh, you seem obsessed'
If they do it in person you can give them a 'you're weirding me out' face. And you can stick with that face and if they say anything to you to go 'oh look, OP is getting offended' or whatever bollocks, you can just quietly go 'its not that... you're just being really weird about it, why do you keep bringing it up?' And if they go 'well, cos you are lying' you can go 'im not, why are you being so weird?' And just repeat that over and over again. The grey rock technique works well with one phrase.